Absolute densities of ground-state atomic oxygen were measured with THz absorption spectroscopy, using the fine structure transition of atomic oxygen at 4.74477749 THz (i.e. 158.268741 cm-1). A tunable THz QCL in continuous-wave mode was used as the radiation source. The THz QCL was operated in a Stirling cryocooler (Ricor K535) at a temperature of either 44.30 or 54.45 K. The laser temperature was regulated by an additional temperature controller (Stanford Research Systems, CTC100), which was capable of a temperature control with <5 mK stability. Tuning of the laser frequency was achieved by linearly ramping the input current. The current was supplied by a laser driver (Wavelength Electronics, QCL1000 OEM) that was controlled by a function generator (Tektronix, AFG3022C) to provide sawtooth waveforms with a frequency of 10 Hz. A phase-locked optical chopper (New Focus, Model 3502) with a frequency of 5 Hz was used to block each second period of the laser output. The generated THz radiation was collected by a gold-coated parabolic mirror to produce a collimated beam with a diameter of approximately 3 mm. A window made of silicon allowed for the laser beam to pass through the plasma reactor, at a height of approximately 18 mm above the RF electrode. A gold-coated mirror at the back end of the reactor reflected the laser beam back towards the entrance window. The effective absorption length was estimated to be 84 cm (i.e. the total length of the laser beam path within the plasma reactor). After passing through the plasma reactor, the laser radiation was detected by a helium-cooled bolometer (Infrared Laboratories, 4.2K Bolometer). The detected signals were recorded with a digital sampling oscilloscope (R&S, RTO 1014).